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The difference in temperatures between immediate parts of a body gives rise to the transfer of heat energy, known as thermal conduction. This heat transfer can occur between different bodies or within the same body.
Heat is transferred through the molecules and electrons in the conducting medium. In simple words, the exchange of energy between the molecules of a body is thermal conduction.
To understand it more clearly, let’s take an example of a metallic rod. Suppose the rod is heated at one end. Then, within a few minutes, heat would flow from one end to the other by the vibrations of electrons present in the rod.
The rate at which this heat flows depends on various factors like the area of the cross-section of the rod, its length, and also on the temperature difference between the two ends. The expression for heat flow is
Therefore, heat flow is directly proportional to the area of cross-section and to the heat difference and inversely proportional to the length of the rod. Also, heat flow always takes place from the higher temperature to the end where the temperature is low, which explains the presence of the minus sign in the above equation.
Modes of transfer of heat energy
There are mainly three modes of heat transfer: convection (for liquids and gases), conduction(for objects in direct contact), and radiation( for objects separated by large spaces).
Metals consist of valence electrons or conduction electrons and, thus, they are good conductors of heat. In solid materials, molecules are tightly bonded, and the spacing between molecules is very small, which makes conduction the most important mode to transfer heat within the body, whereas in liquids and gases, as the spacing between molecules is very large, the heat won’t be transferred through conduction.
Conduction can further be divided into three categories:
- Molecular collisions (in the case of gases and liquids).
- Lattice vibrations for solids.
- Conduction electrons for metals.
Thermal conductivity of metals
Elements that lose electrons to form positive ions are known as metals. The presence of free electrons in the metal makes them very good conductors of heat and electricity. The electrons in metals are bound together by a special type of bonding known as metallic bonding.
The bonded atoms in metals share their valence electrons, which results in a sea of freely flowing electrons, also known as conduction electrons, that carry heat. In non-metals, covalent bonds are present, ie, electrons are shared by two atoms. Therefore, electrons are not free to conduct heat, making them bad conductors of heat and electricity.
Consider an object with a temperature difference between its two ends. Let one end be at temperature T1, and the other be at temperature T2. If T1 is greater than T2, then the heat would flow from T1to T2. The heat flux flowing across a unit area per unit time is given as
J=ΔQ/AΔt=-K dx/dt
Where K is the thermal conductivity in Joule/metre-sec-K.
The amount of heat conducted by an object is directly proportional to its thermal conductivity. Therefore, the more thermal conductivity, the better heat is conducted.
Metals have very high thermal conductivity, and it ranges from 15-450 W/mK at 300K.
Applications of metals as good conductors of heat
Metals find numerous uses in electrical and thermal industries. Metals are used in electronics like phones, laptops, etc, as they are very good conductors of electricity. Copper is used in electrical wiring, and it also conducts heat. The thermal conductivity of aluminium is also very good, making it an excellent conductor of heat and electricity.
The conductivity values for different metals are as follows:
Silver – 429 W/mK
Copper -398 W/mK
Gold -315 W/mK
The filaments of light bulbs are made from tungsten. Tungsten is an excellent conductor of heat. However, tungsten converts most electricity into heat energy, so LEDs have replaced traditional light bulbs. The thermal conductivity of aluminium is also good.
Lastly, the thermal conductivities of metals are required for various purposes, be it safety, efficiency or designing a structure. The value of thermal conductivity varies among metals.
Conclusion
Heat is transferred through the molecules and electrons in the conducting medium. Metals consist of loosely bound electrons or conduction electrons, and thus metals are good conductors of heat.
There are mainly three modes of heat transfer– convection (for liquids and gases), conduction(for objects in direct contact) and radiation (for matters separated by large spaces).
In non-metals, covalent bonds are present, ie, their electrons are shared by two atoms. Therefore, electrons are not free to conduct heat, making them bad conductors of heat and electricity.
The amount of heat conducted by an object is directly proportional to its thermal conductivity. Therefore, the higher the thermal conductivity, the better the heat conduction.
Metal has very high thermal conductivity, and it ranges from 15-450 W/mK at 300K. The thermal conductivities of metals are required for various purposes. The value of thermal conductivity varies among different metals.
